What is a collapse plot?

A plot that has suddenly lost all reason for being.

eg.

"I will follow that villain for the rest of my life and shall not rest until I do him in!"

"Do you mean Mark? Why he died last night in a fight at the nursery. Would you like to see his body?"

"What? Dead? Well, that really put a crimp in my plans. What am I going to do with my life now?"

"Well, there's this sweet young widow that lives just down the lane..."